1. Field of the Invention
The present invention relates to a radiometry system comprising an aperture synthesis type antenna.
It further relates to the application of such a system for hyper-frequency imaging.
The invention more particularly relates to an antenna aboard a satellite and, more particularly, aboard a so-called "Low Earth orbit" or "LEO" satellite, for spatial observation of particular environmental parameters characterizing the earth globe, such as ocean salinity and soil moisture. Those observations and measurements are performed with the help of radiometers operating in hyper-frequency ranges.
2. Prior Art
The radiometers conventionally are equipped with antennas for measuring the radiation emitted by the observed earth globe area. Many antenna types as well as various operating techniques were proposed up to now for these antennas.
On of the major problem raised by antennas operating in the above mentioned frequency ranges is that an aperture of a very large size must be provided, which in particular also implies a large weight.
During the eighties, the use of uni-dimensional aperture synthesis antennas consequently was proposed. The aperture synthesis is made possible by providing a plurality of small-size antenna elements, arranged along a determined space configuration, which is equivalent to a single large-size element. A radiometer including such an antenna was used in the NASA "Electronically Scanned Thinned Array Radiometer" or "NASA ESTAR" project.
More recently, since the early nineties, the feasibility of bi-dimensional aperture synthesis antennas has been studied. As a non-limiting example, the applicant studied a prototype antenna of this type for a radiometer pilot project referenced as "MIRAS". The antenna also includes a plurality of small-size antenna elements. These antenna elements are arranged along the three coplanar branches of a "Y" shaped array. Each branch is 8.3 m long, and their spacing is equiangular, i.e. 120 degrees.
Each branch includes 133 regularly spaced antenna elements. The antenna elements are dual linearly polarized, in quadrature: according to arbitrarily called horizontal and vertical polarizations. Their beam width at half power is 70 degrees. The antenna plane is 31.2 degrees angled with respect to the nadir.
This radiometer is described in the article of M. Martin-Neira and J. M. Goutoule, "MIRAS--A Two-dimensional Aperture-Synthesis Radiometer for Soil-Moisture and Ocean-Salinity Observations", "ESA bulletin", November 1997, pages 95-104.
This radiometer is designed for collecting the flux radiated by the earth globe, by means of an antenna showing the above mentioned features.
To obtain the above mentioned aperture synthesis, a bi-dimensional interferometry is implemented.
In general, the basic measurement performed by an aperture synthesis radiometer consists of measuring a so-called "visibility function". This terminology originally derives from the optical interferometry theory and may be better understood by considering a Young interferometer for which each source generates an output interferogram, which alternately shows maxima and minima. The visibility factor of a sine-wave interferogram is defined as the ratio between the difference and the sum of the maximal and minimal amplitudes. This visibility factor may be defined as equal to the complex coherence degree, which constitutes the primary quantity measured in aperture synthesis.
Although the optic theory cannot, without an adaptation, be transposed from the purely optical field to the hyper-frequency range measurement field, for earth globe surface observation, in particular from the "LEO" satellites, a visibility function can also be defined here. The complex correlation (at zero delay) between each possible pair of antenna elements in the interferometry array gives a visibility function point, at a spatial frequency defined by a particular antenna element base line. Ideally, the visibility function consists of the Fourier transform of the brightness of the observed scene, weighted by the antenna element gain diagram, which can be retrieved from an inverse Fourier transform.
In the case of the "MIRAS" radiometer, those operations are performed by connecting each possible antenna element pair with a receiver of the type described in the "block-diagram" of FIG. 1, appended to the present description. The electronic portion is provided in "Monolithic Microwave Integrated Circuit", or "MMIC" technology. The frequency band retained is the "L" band centered on .lambda.=21 cm.
The output voltage signals of two antenna elements VA.sub.i and VA.sub.j are represented, with both arbitrary indexes i and j being higher than or equal to 1 and lower than or equal to the maximum number of antenna elements.
The VA.sub.i and VA.sub.j signals are preprocessed through an amplifier (not shown). More precisely, each polarization component, H and V, is separately processed, and both components are sequentially transmitted, by means of an also not represented switch. The signals are split along two paths. A first path carries signals, which are directly transmitted to a first frequency converter stage, more precisely a frequency down converter, FI.sub.1. The second path includes a 90 degrees phase shifter, so as to obtain signals in quadrature with the first-path signals. The phase-shifted signals are transmitted to a second frequency converter stage FI.sub.2.
An oscillator OSC generating signals at a 1396 MHZ frequency is provided in order to obtain the frequency conversion. Both signal series are then submitted to a one-bit digitalization, in the AN.sub.1 and AN.sub.2 converters, so as to obtain signals representing the sign of those signals. The output signals are designated as Sign(I.sub.j) and Sign(Q.sub.i) wherein I and Q represent the phase and quadrature signals, respectively.
This output signal pair then is forwarded to first is inputs of a one bit digital correlator Co.sub.i. The latter receives on a second input a signal Sign(I.sub.j) representing the sign of the (non phase-shifted) output signal V.sub.j of the j-indexed antenna element. More precisely, the correlator Co.sub.i comprises two digital multipliers Mp.sub.1 and MP.sub.2, the outputs of which respectively are connected to the integrators It.sub.1 and It.sub.2. The multiplier Mp.sub.1 receives both signals Sign(I.sub.i) and Sign(I.sub.j) and the multiplier Mp.sub.2 receives both signals Sign(Q.sub.i) and Sign(I.sub.j).
In the case of the described example, 8,778 correlators are needed to process all antenna element pairs. Taking into account the frequency down conversion, the data flow speed at the correlator output is low.
To summarize, each correlator performs a complex multiplication followed by an integration of a pair of received signals, wherein each pair corresponds to a base line. The result of each elementary operation allows calculating one of the visibility function points.
When resorting to conventional antennas, using the "L" band which is advantageous for the above mentioned applications leads to very large antenna apertures, of the order of 20 m. For a particular actual antenna area (total area of the element), resorting to the aperture synthesis technology results in an equivalent antenna with a much larger area. This result is the same as if the antenna would include a large number of virtual antenna elements.
In the case of the "MIRAS" radiometer for instance, the antenna elements of each branch of the "Y" are distributed on a single, 8.3 m long, element line. However, the calculations and experiments show that this antenna is equivalent to a six-branch star shaped array, inscribed within a circle with a diameter larger than the length of the "Y" branches. This result is illustrated in FIG. 5 of the above-mentioned article, which the reader could refer to for further details.
The aperture synthesis antenna technology consequently is highly interesting since it allows strongly reducing both the space requirements and the weight. It also offers clear advantages with respect to other antenna embodiments, such as the mechanical scanning mode by rotation around an axis of the support satellite.
This however, as indicated, implies antennas with dual linearly polarized elements in quadrature. Since measuring the visibility function, in the case of aperture synthesis antennas, implies considering all combinations of possible antenna element pairs, two components have to be taken into account: the crossed polarization and the parallel polarization. On the other hand, when considering the global radiometric system, the calculation and experiments show that one of the penalizing paramaters is the ratio between the level of the crossed polarization component and the level of the parallel polarization component. There consequently is a need to improve the inverse ratio, i.e. the ratio between the level of the parallel polarization component and the level of the crossed polarization component.
The aim of the invention is to improve this ratio.